BIO Unit 2 Cellular Respiration

Cellular Respiration

The process of fully oxidizing high-energy food molecules, such as glucose, to harvest chemical energy and produce ATP and NADH.

Overall Cellular Respiration Equation

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy; produces approximately 30–32 ATP per glucose.

Aerobic Respiration

Cellular respiration that consumes oxygen and releases carbon dioxide.

Catabolic Fuel Priority

Cells generally break down carbohydrates first, fats second, and proteins last to produce ATP.

Glucose Oxidation

The process of breaking down glucose, causing its chemical potential energy to decrease while energy is captured to produce ATP.

Oxidation During Cellular Respiration

Carbon in glucose loses electrons, often associated with the removal of hydrogen atoms.

Reduction During Cellular Respiration

Oxygen gains electrons and combines with protons to eventually form water.

Four Stages of Cellular Respiration

Glycolysis, pyruvate oxidation, the citric acid cycle, and oxidative phosphorylation.

Glycolysis

The first stage of cellular respiration; splits and oxidizes one 6-carbon glucose into two 3-carbon pyruvate molecules.

Location of Glycolysis

The cytosol/cytoplasm; glycolysis occurs in nearly all living organisms and does not require oxygen.

Glycolysis Energy Investment Phase

Steps 1–3; uses 2 ATP to phosphorylate glucose.

Glycolysis Cleavage Phase

Steps 4–5; splits the molecule into two 3-carbon glyceraldehyde-3-phosphate (G3P) molecules.

Glycolysis Energy Payoff Phase

Steps 6–10; harvests energy to produce NADH and ATP.

Glycolysis Net Yield

Per glucose: 2 pyruvate, 2 NADH, and 2 net ATP.

Glycolysis ATP Production

Produces 2 net ATP through substrate-level phosphorylation.

Phosphofructokinase

The enzyme regulating glycolysis at step 3 through feedback inhibition.

Glycolysis Regulation by ATP

When ATP levels are high, ATP binds to an allosteric site on phosphofructokinase, changes its shape, and inhibits its activity.

ATP as a Substrate and Regulator

ATP supplies energy for reactions and can also regulate enzymes by binding to an allosteric site.

Pyruvate Oxidation

The second stage of cellular respiration; converts pyruvate into acetyl-CoA, producing NADH and releasing CO₂.

Location of Pyruvate Oxidation

The mitochondrial matrix in eukaryotic cells; pyruvate crosses mitochondrial membranes through carrier proteins.

Pyruvate Dehydrogenase

The enzyme complex that catalyzes pyruvate oxidation.

Pyruvate Oxidation Process

Each 3-carbon pyruvate loses one CO₂, leaving a 2-carbon acetyl group that attaches to coenzyme A.

Pyruvate Oxidation Net Yield

Per glucose: 2 acetyl-CoA, 2 NADH, and 2 CO₂.

Pyruvate Oxidation Regulation

When downstream products are abundant, the pyruvate dehydrogenase complex is phosphorylated, changing its shape and turning it off.

Citric Acid Cycle

The third stage of cellular respiration; breaks down acetyl groups, releases CO₂, and generates electron carriers and ATP.

Location of the Citric Acid Cycle

In the mitochondrial matrix of eukaryotes and the cytosol of prokaryotes.

Citric Acid Cycle Starting Molecule

Oxaloacetate, a 4-carbon molecule that combines with a 2-carbon acetyl group to form citrate.

Citrate

A 6-carbon molecule formed when the 2-carbon acetyl group from acetyl-CoA combines with 4-carbon oxaloacetate.

Oxaloacetate's Role in the Citric Acid Cycle

Combines with an acetyl group at the start of the cycle and is regenerated at the end.

Citric Acid Cycle Net Yield

Per glucose, after two turns: 6 NADH, 2 FADH₂, 2 ATP, and 4 CO₂.

Citric Acid Cycle Regulation

High levels of ATP and NADH inhibit multiple steps of the cycle.

Oxidative Phosphorylation

The fourth stage of cellular respiration; uses the electron transport chain and chemiosmosis to produce approximately 26–28 ATP.

Location of Oxidative Phosphorylation

The inner mitochondrial membrane in eukaryotes or the plasma membrane in prokaryotes.

Electron Transport Chain (ETC)

A series of protein complexes and mobile electron carriers that transfer electrons through sequential redox reactions.

ETC Complexes

Complexes I, II, III, and IV participate in electron transport.

CoQ (Ubiquinone)

A mobile electron carrier that transfers electrons within the electron transport chain.

Cytochrome c (Cyt c)

A mobile electron carrier that transfers electrons within the electron transport chain.

NADH Entry into the ETC

NADH donates its electrons to Complex I.

FADH₂ Entry into the ETC

FADH₂ donates its electrons to Complex II.

ETC Proton Pumps

Complexes I, III, and IV use energy from electron transfer to pump H⁺ from the mitochondrial matrix into the intermembrane space.

Proton-Motive Force

The electrochemical H⁺ gradient across a membrane that stores energy used to produce ATP.

Terminal Electron Acceptor in Aerobic Respiration

Oxygen (O₂); it accepts electrons and combines with protons to form water.

Role of Oxygen in the ETC

Oxygen's high electronegativity allows it to accept electrons at the end of the electron transport chain.

Chemiosmosis

The movement of protons down their electrochemical gradient through ATP synthase, providing energy for ATP production.

ATP Synthase

A rotary enzyme that uses proton flow to change shape and join ADP with inorganic phosphate (Pi) to form ATP.

How ATP Synthase Produces ATP

The flow of H⁺ causes enzyme subunits to rotate, producing conformational changes that allow ADP and Pi to combine into ATP.

Oxidative Phosphorylation Net Yield

Approximately 26–28 ATP per glucose.

Anaerobic Respiration

Respiration that uses an electron acceptor other than oxygen; used by certain prokaryotes in oxygen-poor environments.

Anaerobic Respiration Pathway

Uses glycolysis, pyruvate oxidation, the citric acid cycle, and an electron transport chain.

Alternative Terminal Electron Acceptors

Molecules such as nitrate (NO₃⁻) or sulfate (SO₄²⁻) that can accept electrons instead of oxygen.

ATP Production in Anaerobic Respiration

Produces ATP through chemiosmosis but is less efficient than aerobic respiration.

Why Anaerobic Respiration Is Less Efficient

Oxygen is the most electronegative electron acceptor available in the process described, allowing aerobic respiration to extract more energy.

Fermentation

A process that allows glycolysis to continue when no terminal electron acceptor is available and the electron transport chain stops.

Purpose of Fermentation

Transfers electrons from NADH to an organic molecule, regenerating NAD⁺ so glycolysis can continue.

ATP Yield of Fermentation

Produces a net 2 ATP per glucose through glycolysis.

NAD⁺ Regeneration in Fermentation

Fermentation oxidizes NADH back to NAD⁺, allowing glycolysis to continue producing ATP.

Lactic Acid Fermentation

A fermentation pathway in which pyruvate accepts electrons from NADH and is reduced to lactate, regenerating NAD⁺.

Lactic Acid Fermentation Example

Human muscle cells can use lactic acid fermentation when oxygen availability is insufficient for their energy needs.

Alcohol Fermentation

A fermentation pathway in which pyruvate is converted to acetaldehyde, releasing CO₂; acetaldehyde then accepts electrons from NADH to form ethanol.

Acetaldehyde

A 2-carbon molecule formed from pyruvate during alcohol fermentation; it accepts electrons from NADH and becomes ethanol.

CO₂ in Alcohol Fermentation

Carbon dioxide is released when pyruvate is converted into acetaldehyde.

Alcohol Fermentation Products

Ethanol and carbon dioxide; the pathway also regenerates NAD⁺.

Facultative Anaerobes

Organisms that can switch between aerobic cellular respiration and fermentation depending on whether oxygen is available.

Aerobic Respiration vs. Fermentation

Aerobic respiration uses oxygen and includes the ETC; fermentation does not use an ETC and regenerates NAD⁺ so glycolysis can continue.

Anaerobic Respiration vs. Fermentation

Anaerobic respiration uses an ETC with an alternative terminal electron acceptor; fermentation uses an organic molecule to accept electrons and regenerate NAD⁺.